1. Field of the Invention
This invention relates to the preparation of organohalosilanes. More particularly, this invention relates to a method for increasing the yield of monoorganodichlorosilanes using a method referred to in the art as the "direct process" without either adversely affecting the yield of other desirable organochlorosilanes, particularly the combined yield of monoorganodichlorosilane and the corresponding diorganodihalosilanes, or generating substantial quantities of undesirable inorganic halosilanes and organotrihalosilanes.
2. Description of Relevant Art
The preparation of organohalosilanes by the reaction of an alkyl or aryl halide with silicon metal in the presence of various catalysts is known as the "direct process". The halide portion is typically chlorine, but can also be bromine or iodine.
Preparing halosilanes by reacting silicon with hydrogen chloride has been known since the work of Buff and Wohler in 1857 and Combes in 1896. Application of the direct process to the preparation of organohalosilanes was first disclosed by Rochow and his co-workers, beginning in the mid1940's. The art describes numerous improvements to this direct process.
Rochow and Patnode, U.S. Pat. No. 2,380,996, issued Aug. 7, 1945, and Patnode, U.S. Pat. No. 2,380,997, issued Aug. 7, 1945, disclose the preparation of a contact mass for the direct process. The mass is prepared by firing a mixture of silicon, copper, or other metallic catalysts in a reducing atmosphere. Rochow and Patnode and Patnode also disclose the use of nickel, tin, antimony, manganese, silver, and titanium.
Rochow and Gilliam, U.S. Pat. No. 2,383,818, issued Aug. 28, 1945, discloses the use of contact masses comprising silicon and an oxide of copper. Also, included are copper compounds which are readily converted to the oxides, such as copper nitrate.
An example of more recent art is Chapters 4 and 5 of a text entitled Organohalosilanes by R. J. H. Voorhoeve, published in 1967 by Elsevler.
For various reasons, including cost and availability of starting materials, alkylchlorosilanes, particularly methyl- and ethylchlorosilanes, have become the organohalosilanes most frequently prepared by the direct process. The present invention has therefore been limited to this class of alkylchlorosilanes using the corresponding alkyl chlorides. It should be understood that while preferred embodiments of the present invention are directed primarily to the preparation of certain methylchlorosilanes by reacting methyl chloride and silicon, the invention is not to be so limited.
When methyl chloride, represented by the formula MeCl, and silicon metal are reacted using the catalysts and reaction conditions described in the prior art, the resultant products include but are not limited to MeHSiCl.sub.2, Me.sub.2 SiCl.sub.2, Me.sub.3 SiCl, MeSiCl.sub.3, Me.sub.2 HSiCl, HSiCl.sub.3 and SiCl.sub.4, where Me represents the methyl radical. By an appropriate selection of catalyst and reaction conditions it is possible to obtain dimethyldichlorosilane, Me.sub.2 SiCl.sub.2, as the major component, often 90 weight % or more, in the final product mixture. Methyldichlorosilane, MeHSiCl.sub.2 typically constitutes about 1 weight percent of the product under these conditions, which are designed to optimize the yield of Me.sub.2 SiCl.sub.2. The reaction product also typically contains a significant concentration of methyltrichlorosilane, MeSiCl.sub.3, which in some instances can be a desired product.
By contrast, the inorganic tetra- and trichlorosilanes, SiCl.sub.4 and HSiCl.sub.3 are almost always undesirable products of the direct process for preparing methylchlorosilanes. Not only do these inorganic silanes reduce the amount of silicon converted to useful products, but they are difficult to separate from the desired methylchlorosilanes, thereby further increasing the cost of preparing the desired silanes.
If one desires to increase the relative amount of methyldichlorosilane in a direct process product mixture without either a substantial decrease in the combined yield of this product and dimethyldichlorosilane or producing substantial amounts of undesirable products, particularly the inorganic chlorosilanes and, in many instances MeSiCl.sub.3, the prior art provides no route for achieving this objective. This art does teach combining the methyl chloride with 5 weight percent or more, based on methyl chloride, of hydrogen chloride as a means for increasing the relative yield of methyldichlorosilane in the final product, however the yield of dimethyldichlorosilane is more than correspondingly reduced and substantial quantities of undesirable inorganic halosilanes are produced. For example, Ariga et al U.S. Pat. No. 3,454,616, which issued on July 8, 1969 teaches reacting silicon metal with mixtures of methyl chloride and from 20 to 83 percent, based on the weight of the mixture, of hydrogen chloride. In accordance with the examples of this patent, when these gaseous mixtures are reacted with metallic silicon containing catalytic amounts of copper and nickel and reaction product contains up to 36 weight percent of methyldichlorosilane. Depending upon reaction conditions and the molar ratio of hydrogen chloride to methyl chloride, the reaction product also contained from 2.4 to 32.5 weight percent of dimethyldichlorosilane, from 5.6 to 33 weight percent of the highly undesirable trichlorosilane and from 0.6 to 2.3 weight percent of SiCl.sub.4.
The addition of aluminum chloride or boron trichloride in catalytic amounts to increase the amount of methyldichlorosilane formed by the reaction of methyl chloride and hydrogen chloride with silicon metal is taught in Tamura et al. U.S. Pat. No. 3,109,014, which issued on Oct. 29, 1963. As in the Ariga et al. patent, excessive amounts of the undesirable inorganic halosilanes are produced.
Golubsov et al. report in the Journal of Applied Chemistry, Russian edition, 37 (7), p. 1634 (1964) that the presence of hydrogen chloride increases the yield of phenyltrichlorosilane from the reaction of chlorobenzene and silicon metal. A product containing 55 mole percent of phenyltrichlorosilane and only 0.7 mole percent of phenyldichlorosilane is reportedly obtained from the reaction of a silicon alloy with a mixture of chlorobenzene and hydrogen chloride containing 62 weight percent of hydrogen chloride. The yield of diphenyldichlorosilane is not disclosed.
L. Morozova et al. [Izvestia Akaademii Nauk SSSR Ser. Kim. (1962) (6) 941] reacted a mixture of silicon and catalytic amounts of copper oxide, zinc oxide and sodium silicate with a mixture of methyl chloride and 5 or 10 percent by volume of hydrogen chloride at a temperature of 350.degree. C. Five volume percent of hydrogen chloride produced 43 weight percent of methyldichlorosilane, 2 percent of dimethyldichlorosilane, 31 percent of methyltrichlorosilane, and 6 weight percent of silicon tetrachloride. Ten volume percent of hydrogen chloride yielded 42 weight percent of methyldichlorosilane, 8 percent of methyldichlorosilane and 9 percent of silicon tetrachloride.
The data in an article by Gorbunov et al. in the Sept. 1970 issue of Doklady Akademi Nauk. SSR. 194, 1 (92-94) demonstrate that 35 weight percent of methyldichlorosilane, only about 10 weight percent of dimethyldichlorosilane with about the same amount of silicon tetrachloride is obtained by reacting silicon with a 3:1 weight ratio mixture of methyl chloride and hydrogen chloride at 300 degrees C.
J. Joklik et al. [Collect. Czech. Chem. Commun. (1964) 29(3) 834] disclose reacting silicon with various ratios of methyl chloride to hydrogen chloride in the presence of a copper catalyst at temperatures of 260, 300, and 350.degree. C. At 260.degree. C. as the percent by volume of hydrogen chloride in the hydrogen chloride/methyl chloride mixture was increased from 14 to 40 percent and the weight percent of methyldichlorosilane in the reaction product increased from 8.9 to 14 percent under these conditions the yield of trichlorosilane increased from 4.3 to 15 percent and the yield of MeSiCl.sub.3 increased from 25.5 to 32.9%. At 350.degree. C. the yields of methyldichlorosilane, trichlorosilane and silicon tetrachloride were 6.5, 0.4 and 0.3 percent by weight, respectively using 14 volume percent of hydrogen chloride and produced 35.3 weight percent MeSiCl.sub.3. When the volume percent of hydrogen chloride was increased to 40 percent the reaction product contained 14.1 weight percent of methyldichlorosilane, 2.9 percent of trichlorosilane and 1.2 percent of silicon tetrachloride.
Finally, the effect of diluting ethyl chloride with various levels of hydrogen chloride was reported on by Andrianov et al. in Izvestiya Akademii Nauk SSSR (Chemical Section) 10, 1788-1794 (1962). Reacting a mixture of 92 weight percent ethyl chloride and 8 percent hydrogen chloride yielded a mixture of chlorosilanes containing 30 percent by weight of trichlorosilane, 12 percent of SiCl.sub.4, 8.8 percent of ethyldichlorosilane and 50 percent of diethyldichlorosilane.